In some cases, channels for wireless local area network (WLAN) transmission opportunity (TXOP) functionality are reserved for a certain transmitter-receiver pair for a defined time period, such as in Institute of Electrical and Electronics Engineers (IEEE™) 802.11-based networks that otherwise operate according to Distributed Coordinated Function (DCF) rules. In such networks, an access point (AP) or station (STA) that has gained the TXOP is called the TXOP holder and it sends frames to another STA or AP, which is the TXOP responder. During these TXOPs, multiple frames can be transmitted sequentially, spaced by a Short Interframe Space (SIFS) duration. Since upon obtaining a TXOP, the TXOP holder can transmit multiple packets one after the other without the need to contend again for medium access, TXOP operation can significantly improve the system capacity when compared to DCF-based channel access. There are several types of TXOPs that can be obtained, such as, for example, Hybrid Coordination Function (HCF) Enhanced Distributed Channel Access TXOPs (EDCA TXOPs) and HCF Control Channel Access TXOPs (HCCA TXOPs) in a Basic Service Set (BSS) network, HCF EDCA TXOPs in an Independent Basic Service Set (IBSS) network, and TXOP operation in a Wireless Mesh Network (WMN).
In HCF, the Hybrid Coordinator (HC) controls the bandwidth by allocating multiple transmission opportunities to the STAs. TXOPs are obtained either by regular channel contention in EDCA or by the Hybrid Coordinator in HCCA. During a mesh peer service period, a mesh STA may obtain multiple TXOPs in a single direction. Similarly, other network operation modes may reserve the channel for multiple transmissions that are spaced apart by a given duration.
In addition, Hybrid Automatic Repeat Request (HARQ) protocol has been used in cellular systems when a transmission is contention free, meaning that each transmitter and receiver pair has dedicated resources to be used for transmission. With dedicated resources, a receiver is able to assume that it is the intended recipient of any received transmission and retransmission. In such cases, even if a transmitted packet is not decoded correctly, the receiver may be configured to keep the received information of the transmitted packet in its memory and send a Negative Acknowledgement (NACK) to the transmitter to request re-transmission. During the reception of the retransmission, the receiver may use both the first transmission and the re-transmission(s) by combining them together based on the configured HARQ to decode the packet correctly.
HARQ principles referred to as cache-combining and incremental redundancy may be used in cellular systems. In cache-combining, each transmitter packet may be self-decodable and the receiver is configured to sum the signal energy of each transmission. In incremental redundancy, a retransmission contains additional redundancy information to improve Forward Error Correction (FEC) and channel coding results.
In this fashion, HARQ can employ link adaptation more aggressively, since the effect of packet transmission failure is smaller and the combination of separate transmissions is possible. For example, a typical operation point of link adaptation with HARQ is 10% of Block Error Rate (BLER), where 90% of transmissions go through with an aggressive Modulation and Coding Scheme (MCS) setting and the remaining 10% is recovered with relatively few re-transmissions.
Due to the way that channels are allocated in existing 802.11-based networks and the fact that a transmission and its retransmission(s) are completely independent of each other, HARQ protocols have not been implemented in contention-based transmissions.